U.S. Senate Committee on Environment & Public Works
Hearing Statements

Date: 07/29/2003

Dr. David R. Legates
Director, Center for Climatic Research
University of Delaware
Climate history and mercury emissions

Distinguished Senators, panelists, and members of the audience: I would like to thank the Committee for inviting my commentary on this important topic of climate history and its implications. My name is David R. Legates and I am an Associate Professor and Director of the Center for Climatic Research at the University of Delaware in Newark, Delaware. My research interests have focused on hydroclimatology – the study of water in the atmosphere and on the land – and on the application of statistical methods in climatological research.

I am familiar with the testimony presented here by Dr. Soon. I agree with his statements and I will not reiterate his arguments. My contributions to Dr. Soon’s research stemmed from my grappling with the apparent tautology between the long-standing historical record and the time-series recently presented by Dr. Mann and his colleagues. It also stems from my own experiences in compiling and merging global estimates of air temperature and precipitation from a variety of disparate sources.

My Ph.D. dissertation resulted in the compilation of high-resolution climatologies of global air temperature and precipitation. From that experience, I have become acutely aware of the issues associated with merging data from a variety of sources and containing various biases and uncertainties. By its very nature, climatological data exhibit a number of spatial and temporal biases that must be taken into account. Instrumental records exist only for the last century or so and thus proxy records can only be used to glean information about the climate for earlier time periods. But it must be noted that proxy records are not observations and strong caveats must be considered when they are used. It too must be noted that observational data are not without bias either.

The Historical Record of the Last Millennium

Much research has described both the written and oral histories of the climate as well as the proxy climate records (e.g., ice cores, tree rings, and sedimentations) that have been derived for the last millennium. It is recognized that such records are not without their biases – for example, historical accounts often are tainted with the preconceived beliefs and limited experiences of explorers and historians while trees and vegetation respond not just to air temperature fluctuations, but to the entire hydrologic cycle of water supply (precipitation) and demand (which is, in part, driven by air temperature). Nevertheless, such accounts indicate that the climate of the last millennium has been characterized by considerable variability and that extended periods of cold and warmth existed. It has been generally agreed that during the early periods of the last millennium, air temperatures were warmer and that temperatures became cooler toward the middle of the millennium. This gave rise to the terms the “Medieval Warm Period” and the “Little Ice Age”, respectively. However, as these periods were not always consistently warm or cold nor were the extremes geographically commensurate in time, such terms must be used with care.

A Biased Record Presented by the IPCC and National Assessment

In a change from its earlier report, however, the Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC), and now the US National Assessment of Climate Change, both indicate that hemispheric or global air temperatures followed a curve developed by Dr. Mann and his colleagues in 1999. This curve exhibits two notable features. First is a relatively flat and somewhat decreasing trend in air temperature that extends from 1000AD to about 1900AD and is associated with a relatively high degree of uncertainty. This is followed by an abrupt rise in air temperature during the 1900s that culminates in 1998 with the highest temperature on the graph. Virtually no uncertainty is shown for the data of the last century. The conclusion reached by the IPCC and the National Assessment is that the 1990s are the warmest decade with 1998 being the warmest year of the last millennium.

Despite the large uncertainty, the surprising lack of variability in the record gives the impression that climate remained relatively unchanged through most of the last millennium – at least until human influences began to cause an abrupt increase in temperatures during the last century. Interestingly, Mann et al. replace the proxy data for the 1900s by the instrumental record and no uncertainty characterization is provided. This too yields a false impression that the instrumental record is consistent with the proxy data and that it is ‘error free’. It is neither. The instrumental record contains numerous uncertainties, resulting from a lack of coverage over the world’s oceans, an under-representation of mountainous and polar regions as well as under-developed nations, and the presence of urbanization effects resulting from the growth of cities. Even if a modest uncertainty of a ±0.1°C were imposed on the instrumental record, the claim of the 1990s being the warmest decade would immediately become questionable, as the uncertainty window would overlap with the uncertainty associated with earlier time periods. Note that if the satellite temperature record – where little warming has been observed over the last twenty years – had been inserted instead of the instrumental record, it would be impossible to argue that the 1990s are the warmest decade.

Rationale for the Soon et al. Investigation

So we were left to question why the Mann et al. curve seems to be at variance with the previous historical characterization of climatic variability. Investigating more than several hundred studies that have developed proxy records, we came to the conclusion that nearly all of these records show considerable fluctuations in air temperature over the last millennium. Please note that we did not reanalyze the proxy data – the original analysis from the various researchers was left intact. Most records show the coldest period is commensurate with at least a portion of what is termed the “Little Ice Age” and the warmest conditions are concomitant with at least a portion of what is termed the “Medieval Warm Period”.

But our conclusion is entirely consistent with conclusions reached by Drs. Bradley and Jones that not all locations on the globe experienced cold or warm conditions simultaneously. Moreover, we chose not to append the instrumental record, but to compare apples-with-apples and determine if the proxy records themselves indeed confirm the claim of the 1990s being the warmest decade of the last millennium. That claim is not borne out by the individual proxy records.

However, the IPCC report, in the chapter with Dr. Mann as a lead author and his colleagues as contributing authors, also concludes that research by Drs. Mann, Jones, and their colleagues “support the idea that the 15th to 19th centuries were the coldest of the millennium over the Northern Hemisphere overall.” Moreover, the IPCC report also concludes that the Mann and Jones research “show[s] temperatures from the 11th to 14th centuries to be about 0.2°C warmer than those from the 15th to 19th centuries.” This again is entirely consistent with our findings. Where we differ with Dr. Mann and his colleagues is in their construction of the hemispheric averaged time-series, their assertion that the 1990s are the warmest decade of the last millennium, and that human influences appear to be the only significant factor on globally averaged air temperature. Reasons why the Mann et al. curve fails to retain the fidelity of the individual proxy records are detailed statistical issues into which I will not delve. But our real difference of opinion focuses solely on the Mann et al. curve and how we have concluded it misrepresents the individual proxy records. In a very real sense, this is an important issue that scientists must address before the Mann et al. curve is taken as fact.

Our work has been met with much consternation from a variety of sources and we welcome healthy scientific debate. After all, it is disagreements among scientists that often lead to new theories and discoveries. However, I am aware that the editors of the two journals that published the Soon et al. articles have been vilified and the discussion has even gone so far as to suggest that Drs. Soon and Baliunas be barred from publishing in the journal Climate Research. Such tactics have no place in scientific debate and they inhibit the free exchange of ideas that is the hallmark of scientific inquiry.

Climate is More Than Mean Global Air Temperature

In closing, let me state that climate is more than simply annually-averaged global air temperature. Too much focus has been placed on divining air temperature time-series and such emphasis obscures the true issue in understanding climate change and variability. If we are truly to understand climate and its impacts and driving forces, we must push beyond the tendency to distill it to a single annual number. Proxy records, which provide our only possible link to the past, are incomplete at best. But when these records are carefully and individually examined, one reaches the conclusion that climate variability has been a natural occurrence, and especially so over the last millennium. And given the uncertainties in the proxy and instrumental records, an assertion of any decade as being the warmest in the last millennium is premature.

I’m sorry that a discussion that is best conducted among scientists has made its way to a United States Senate committee. But hopefully a healthy scientific debate will not be compromised and we can push on towards a better understanding of climate change.

I again thank you for the privilege of speaking before you today.

DAVID R. LEGATES, Ph.D, C.C.M July 2003

Home Address: 111 Dawn Drive, Middletown DE 19709

Campus Address: Center for Climatic Research, University of Delaware, Newark DE 19716

Legates, D.R. (1997). Comments on “Global and Terrestrial Precipitation: A Comparative Assessment of Existing Climatologies” — A Reply. International Journal of Climatology, 17:779–783.

Legates, D.R., K.R. Nixon, T.D. Stockdale, and G.E. Quelch (1996). Soil Water Management Using a Water Resource Decision Support System and Calibrated WSR-88D Precipitation Estimates. Symposium on GIS and Water Resources, American Water Resources Association, 427–435.

Groisman, P.Ya., and D.R. Legates (1995). Documenting and Detecting Long-Term Precipitation Trends: Where We Are and What Should be Done. Climatic Change, 31:601–622.

Legates, D.R., T.L. DeLiberty, and J.M. Salisbury (1994). Implications of Doubled Trace Gas Concentrations on Summer Precipitation Variability in the Southern Great Plains. Symposium on the Effects of Human-Induced Changes on Hydrologic Systems, American Water Resources Association, 755–762.

Groisman, P.Ya., and D.R. Legates (1994). Accuracy of Historical United States Precipitation Data. Bulletin of the American Meteorological Society, 75(2):215–227.

Legates, D.R. (1993). The Effect of Domain Shape on Principal Components Analyses: A Reply. International Journal of Climatology, 13:219–228.

Legates, D.R., and T.L. DeLiberty (1993). Measurement Biases in the United States Raingage Network. Symposium on Geographic Information Systems and Water Resources, American Water Resources Association, 547–557.